mutations in somatic cells is the primary cause of aging. A comparison of somatic mutation rate across several mammal species found that the total number of accumulated mutations at the end of lifespan was roughly equal across a broad range of lifespans. The authors state that this strong relationship between somatic mutation rate and lifespan across different mammalian species suggests that evolution may constrain somatic mutation rates, perhaps by selection acting on different DNA repair pathways. Antagonistic pleiotropy Medawar's theory was critiqued and later further developed by George C. Williams in 1957. Williams noted that senescence may be causing many deaths even if animals are not 'dying of old age.' He began his hypothesis with the idea that ageing can cause earlier senescence due to the competitive nature of life. Even a small amount of ageing can be fatal; hence natural selection does indeed care and ageing is not cost-free. Williams eventually proposed his own hypothesis called antagonistic pleiotropy. Pleiotropy, alone, means one mutation that cause multiple effects on phenotype. Antagonistic pleiotropy on the other hand deals with one gene that creates two traits with one being beneficial and the other detrimental. In essence, this refers to genes that offer benefits early in life, but later accumulate a cost. In other words, antagonistic pleiotropy is when the resultant relationship between two traits is negative. It's when one phenotypic trait positively affects current reproduction at the expense of later accelerated senescence, growth, and maintenance. Antagonistic pleiotropy is permanent unless a mutation that modifies the effects of the primary locus occurs. Although antagonistic pleiotropy is a prevailing theory today, this is largely by default, and has not been well verified. Research has shown that this is not true for all genes and may be thought of as partial validation of the theory, but it cuts the core premise: that genetic trade-offs are the root cause of ageing. In breeding experiments, Michael R. Rose selected fruit flies for long lifespan. Based on antagonistic pleiotropy, Rose expected that this would surely reduce their fertility. His team found that they were able to breed flies that lived more than twice as long as the flies they started with, but to their surprise, the long-lived, inbred flies actually laid more eggs than the short-lived flies. This was another setback for pleiotropy theory, though Rose maintains it may be an experimental artifact. Disposable soma theory A third